1. Field of the Invention
This invention relates to a high-power semiconductor laser device in which the density of laser light within the active layer is reduced in the vicinity of the cavity facets.
2. Description of the Prior Art
In recent years, semiconductor laser devices have been widely used as a light source for optical disk players and optical communication systems. The semiconductor laser devices used for these applications are required to emit laser light with high output power. However, the output power of semiconductor laser devices which have been put to practical use is only as high as 50 mW. One possible reason that high output power cannot be attained is deterioration of crystals at the cavity facets. To reduce the crystal deterioration, liquid phase epitaxy (LPE) is used to make the thickness of an active layer smaller in the vicinity of the facets than in the inside of the device, thereby reducing the density of laser light within the active layer in the vicinity of the facets.
FIG. 5c shows a conventional semiconductor laser device with a T.sup.3 structure which is well known as a device of this type (see, e.g., Technical Report of Mitsubishi Denki Co., Ltd., Vol. 62, No. 7, 14(566), 1988). FIG. 5d is a top plan view showing the semiconductor laser device, in which a ridge configuration is indicated by broken lines. FIGS. 5a and 5b are perspective views showing the production of the semiconductor laser device.
The semiconductor laser device of FIG. 5c is produced as follows: On the plane of a p-GaAs substrate 21, a ridge portion 32 with a height of 2 .mu.m is formed by an etching technique. The width of the ridge portion 32 is small in the vicinity of the facets and great in the inside of the device, as shown in FIG. 5a. Then, on the entire surface of the substrate 21 including the ridge portion 32, an n-GaAs current blocking layer 22 is formed by an epitaxial growth method and on the center of the ridge portion 32, a V-striped groove 31 is formed through the current blocking layer 22 by an etching technique, as shown in FIG. 5b. Moreover, on the entire surface of the current blocking layer 22 including the V-striped groove 31, a p-AlGaAs first cladding layer 23 an AlGaAs active layer 24, an n-AlGaAs second cladding layer 25, and an n-GaAs cap layer 26 are successively grown by liquid phase epitaxy, resulting in a double-heterostructure shown in FIG. 5c.
In cases where crystal growth on the substrate having the ridge portion is conducted by liquid phase epitaxy as described above, the crystal growth is promoted on the side faces of the ridge portion, whereas the crystal growth is suppressed on the upper face of the ridge portion. This phenomenon is well known as an anisotropy of crystal growth and this property causes a significantly thinner crystal film to be grown on the upper face than on the side faces of the ridge portion.
With the use of the anisotropy of crystal growth, the thickness of a layer formed on the ridge portion can be controlled by a change in the width of the ridge portion. That is, the grown layer becomes thick in the wide region but becomes thin in the narrow region of the ridge portion. Therefore, when several layers including the active layer 24 are formed on the ridge portion 32 of a configuration shown in FIG. 5a by liquid phase epitaxy, the active layer 24 becomes thin in the vicinity of the facets but becomes thick in the inside of the device. In the T.sup.3 type semiconductor laser device produced in such a manner, because the active layer 24 is thin in the vicinity of the facets, part of laser light enters into the first cladding layer 23 and the second cladding layer 25 in the vicinity of the facets, thereby reducing the density of the laser light within the active layer 24. Thus, optical output power at the limit of destruction can be improved to attain high-power operation.
In the T.sup.3 type semiconductor laser device as produced above, the first cladding layer 23 should be grown prior to the growth of the active layer 24 and serious problems arise therefrom.
The first cladding layer 23 is thin in the vicinity of the facets and thick in the inside of the device because of the growth by liquid phase epitaxy. Such a difference in the thickness of the first cladding layer 23 is not preferred, because the thickness of the first cladding layer 23 should be set at a value suitable for the formation of an appropriate index guiding mechanism.
The active layer 24 is not also flat and has irregularities as large as 0.2 .mu.m, because it is formed on the uneven first cladding layer 23 by liquid phase epitaxy. This situation is shown in a sectional view of FIG. 6, which is taken at line VI--VI of FIG. 5d. Thus, the irregularities of the active layer 24 cause the occurrence of sub-peaks in the vertical direction of the far field pattern of emitted light, resulting in a deterioration of the device characteristics.